1. Technical Field
This invention relates to aqueous free-flowing compositions. In particular, the invention relates to surfactant compositions that have free-flowing non-Newtonian shear thinning properties and the ability to suspend components.
2. Background of Related Art
It has been documented that free-flowing non-Newtonian shear thinning personal care preparations, comprised of well defined surfactant mixtures, are capable of suspending water-insoluble particles or partially insoluble components, such as vegetable oils, mineral oils, silicone oils, solid particles, abrasives, and similar articles. Examples of such preparations can be found in U.S. Pat. Nos. 5,556,628 and 5,965,500, each of which are incorporated by reference herein to the extent they are not inconsistent with this application. These systems provide a means to include otherwise difficult to incorporate components in surfactant mixtures resulting in cosmetic preparations with multi-functional benefits including, in some cases, cleansing, moisturizing, improved skin feel, exfoliation/abrasion, novel appearance, or a combination of these benefits.
The rheological behavior of all surfactant solutions, including liquid personal care solutions, is believed to be dependent on their microstructure, i.e., the shape and concentration of micelles or other self-assembled structures in solutions.
Micelles are not necessarily spherical and may, for example, exist as cylindrical or discoidal micelles. At higher concentrations more ordered liquid crystal phases such as lamellar phases, hexagonal phases or cubic phases may form. Surfactants can take on organized phases above the critical micelle concentration or CMC. (The CMC is defined as the concentration of a surfactant at which it begins to form micelles in solution.) The rheology of the phase is very important when considering the usefulness of a surfactant system.
The rheology of surfactant systems can be described in terms of Newtonian and non-Newtonian viscosities. The rheology of a Newtonian surfactant is described as having a viscosity that is independent of the shear rate (i.e., the system will have the same viscosity as different levels of shear are applied). The rheology of a non-Newtonian surfactant system is described as having a viscosity that is dependent on the shear rate. For a non-Newtonian shear thinning surfactant system, viscosity will be reduced as shear rate is increased. This non-Newtonian Theological behavior effectively allows the suspension of undissolved solids, liquids and gases.
According to U.S. Pat. No. 5,556,628, free flowing non-Newtonian shear thinning cosmetic preparations with good storage stability properties can be prepared utilizing specified surfactant mixtures comprised of an anionic surfactant, sodium lauryl ether sulfate (also known as sodium laureth sulfate) and identified co-surfactants and electrolytes. Experiments confirm that non-Newtonian shear thinning formulations can be prepared following the teachings of U.S. Pat. No. 5,556,628. The subject formulations demonstrate good room temperature (25° C.) and elevated temperature (45° C.) viscosity stabilities. However, such systems may not exhibit optimum performance under all conditions including, but not limited to, freeze/thaw conditions.
Other disclosures suggest the use of fatty acid structurants to stabilize lamellar phase systems (see e.g., U.S. Pat. Nos. 6,150,312, 5,952,286 and 5,962,395). The inherent disadvantage of such systems requiring fatty acid ingredients is that fatty acids form insoluble salts (Ca+2 and Mg+2 salts) in hard water, which leave an undesirable residue on surfaces such as hair, skin, hard surfaces, etc. This residue is particularly unwanted in shampoo formulations where it will cause dulling of the hair and will act as a foam depressant that negatively impacts high foaming cosmetic formulations such as hair shampoos and body washes.
Moreover, it is difficult to maintain the stability and integrity of other known systems, particularly under freeze/thaw conditions.
Accordingly, it would be desirable to obtain a free flowing non-Newtonian shear thinning composition suitable for use in personal care compositions having the capacity to suspend water-insoluble particles and having stability under freeze/thaw conditions.